The pathobiont Pg is the bacteria detected most often in patients with both PD and RA, especially in smokers; smoking also strongly worsens PD and RA. Several lines of evidence suggest that Pg might contribute to some PD and RA pathogenesis: Pg produces cysteine proteases, such as gingipain RgpA, endowed with the potential to induce significant bone loss in animal model systems and in patients in either alveolar or subchondral bone. This evidence concerns the gene CTSB and rheumatoid arthritis.